The advantages with lignin separation from black liquor is already described in WO 2006/031175 and WO2006/038863. These patents disclose the novel process LignoBoost™ that is now sold by Metso, and wherein WO 2006/031175 disclose the basic two stage acidic wash process and WO2006/038863 disclose an improvement of the process where sulphate or sulphate ions are added to the process.
An important aspect of the process is that the required charge of chemicals for the acidification may be high. If this is the case the cost of fresh chemicals is a large part of the operational cost and the commercial viability of the process is lower.
These problems could be reduced, if the process is optimized for minimum requirement for charges of fresh chemicals, making the lignin product commercially sound. Acidifiers in form of mill generated waste flows are thus preferable as it may solve a waste disposal problem and lessen environmental impact. As the precipitation of lignin requires acidification of alkaline black liquor flows, much of the total amount of acidifier is used to decrease the pH level down to the point of where lignin starts to precipitate. The first phase reaching this pH level typically reduce the pH level from about pH 13 in the original black liquor down to a pH level about 11.5, and normally do not involve any nucleation of lignin particles. The amount of acidifier needed is nevertheless relatively high for this first phase as the pH follows a logarithmic scale, and any following additional lowering of pH from 11.5 requires far less acidifier for the same order of lowered absolute pH value.
The Lignoboost process produce a lignin product which if used as fuel is classified as a “green” fuel as being based upon recovered fuel. The idea with classification of “green” fuels is based upon the concept not to increase the carbon dioxide footprint, i.e. the emissions, by burning fossil fuels. The most promising acids for this process is carbon dioxide for at least initial precipitation of the lignin, and then using sulfuric acid (H2SO4) for washing and leaching out metals from the lignin. The sulfuric acid could be added as a fresh sulfuric acid from a chemical supplier, or as preferred using so called “spent acid” from a chlorine dioxide generator often used at a pulp mill. The latter usage of this spent acid already at hand in most mill sites further emphasize that the lignin product is considered as a “green” fuel.
One disadvantage with using sulfuric acid is that the waste liquids from the washing process of the lignin contains a lot of sulphur and if these waste liquids are recycled to the recovery process they will end up in the recovery boiler ashes. The sulphur balance of the mill will be affected and in order to reestablish the correct sodium/sulphur balance in the cooking chemicals must fresh sodium be added at high costs. As no better acid for lignin washing has been found that match the results from using sulfuric acid as of lignin yield and practical filterability of the lignin product, a need to solve the sulphur balance of the mill is given priority.
The invention is based upon the finding that the used acidic filtrate from the acidic wash process of the precipitated lignin may be subjected to a sulphur removal process that bleed outs the sulphur in form of gypsum. By this sulphur removal process is the amount of additional sulphur recycled to the recovery process reduced significantly solving the problem with the sulphur balance.
Thus, the invention is related to a method for separation of lignin from original black liquor having a first pH value, comprising the following phases in sequence:                a first precipitation stage wherein an acidifier charge is added to the original black liquor in order to decrease the pH value of the original black liquor to a second pH level initiating precipitation of lignin whereby said second pH level is above pH7 and below 11.5,        followed by a separation phase wherein the precipitated lignin is separated as a lignin cake from the remaining liquid phase of the acidified original black liquor,        suspending the lignin cake in a strong acid solution establishing a pH level below 5 and maintaining the acidified lignin slurry in this slurry state for at least 2 minutes,        dewatering the acidified lignin slurry obtaining a second lignin cake and at least one acidic liquid phase,        and according to the inventive aspects is the acidic liquid phase subjected to a sulphur removal process wherein a calcium containing compound is added to the acidic liquid phase, whereby sulphur in the acidic liquid phase is reacting with the calcium compound forming solid gypsum,        separating the solid gypsum from the acidic liquid phase and sending the remaining acidic liquid phase with reduced sulphur content to chemical recovery.        
By this method could the sulphur containing acidic liquid phase be depleted from its content of sulphur in an efficient manner solving the problem with the sulphur balance of the mill, while the formed gypsum may be expelled from the process.
Preferably is the calcium containing compound calcium carbonate and that carbon dioxide is formed during the reaction and wherein at least a part of the carbon dioxide released in the sulphur removal process is used as part of the acidifier charge added to the original black liquor. By this usage of carbon dioxide generated in the process itself could the need for external supply of acidifier for the precipitation process be reduced considerably such that the lignin extraction process becomes a self sustaining process generating its own necessary chemicals.
As an alternative to the calcium containing compound could calcium acetate be used and that carbon dioxide is formed during the reaction and wherein at least a part of the carbon dioxide released in the sulphur removal process is used as part of the acidifier charge added to the original black liquor.
According to yet another aspect of the invention is acid added to the sulphur removal process in order to maintain the sulphur removal process acidic and decrease the solubility of gypsum. More of the sulphur content may thus be bound in gypsum formed. Preferably is the sulphur removal process kept at a pH in the range 4-8, and more preferably about pH 5. The acid added to the sulphur removal process is preferably an organic acid and among these organic acids are acetic acid, formic acid or citric acid found to be preferred. The organic acid could be mixed with the calcium containing compound before being added to the sulphur removal process.
Calcium carbonate could for example be soaked in organic acid in a first step, forming calcium acetate, and then thereafter adding the calcium acetate.
In order to not loose sodium with the separated solid gypsum is also preferred to wash out any residual sodium from the solid gypsum and return the wash filtrate to the evaporation stages.
It is intended throughout the present description that the expression “dewatering” embraces any means of dewatering. Preferably the dewatering is performed by using centrifugation, a filter press apparatus, a band filter, a rotary filter, such as a drum filter, or a sedimentation tank, or similar equipment, most preferred a filter press apparatus is used.
It is intended throughout the present description that the expression “original black liquor” embraces spent cooking liquor from a digester, having most of the lignin from the original cellulose material dissolved in the “original black liquor”. The “original black liquor” may also have a large content of organic and inorganic material, but may also have passed through separation processes for extracting turpentine or other specific constituents, while keeping the bulk volume of dissolved lignin unaltered.
It is intended throughout the present description that the expression “lime kiln” embraces the conversion plant in the recovery island where the calcium carbonate in the lime mud obtained in the recaustizising plant is calcined to calcium oxide and reused in the lime cycle.